Orthodontic expansion screws may be used for the generation of an expansion force within the upper jaw of a patient. Expansion screws may be used for separation of the palatal suture for the skeletal widening of the upper jaw. For example, an expansion screw may widen a narrow upper jaw by opening the palatine suture by applying force to straps attached to opposite molars of the upper jaw. As shown in FIGS. 1A-D, an expansion screw 100 may comprise a first body 102 and second body 104. As shown the bodies are rectangular in the frontal plane views of FIGS. 1A and 1C, and rectangular in the transverse plane views if FIGS. 1B and 1D. The first body 102 and the second body 104 are movable via a spindle 106 threadably coupled to the bodies. Supporting wires 108 are coupled to molar bands 110 on opposite sides of the expansion screw 100. FIG. 1A, shows a transverse plane view and FIG. 1B shows a frontal plane view of expansion screw 100 in a first un-spread configuration. By increasing the distance between the first and second bodies pressure between the molar bands attached to maxillary molars 112 is created which is transmitted to the upper jaw. FIG. 1C, shows a transverse plane view and FIG. 1D shows a frontal plane view of expansion screw 100 in a second spread configuration, wherein the distance between the bodies in greater than the distance in the first configuration. Increasing the distance, and therefore the pressure, between the maxillary molars by increasing the distance between the bodies with the spindle will lead to the palatine suture breaking open, widening of the midpalatal suture which will widen the upper jaw. In addition to this mechanism of breaking open and widening the midpalatal suture, expansion screws may further be used to widen an upper jaw via other mechanisms and/or may be used to move a single tooth of group of teeth in either jaw.
As shown in FIGS. 1A-1D, the use of existing expansion screws may lead to portions of the expansion screw impinging palate tissue 114. This impingement may cause discomfort to the patient and it may lead to prolonged treatment due to temporary removal of the expansion screw in order to ease the discomfort. Specifically, as shown in FIG. 1B, in the first configuration the bodies 102 and 104 do not contact the palate tissues 114. With the distance between the bodies increased to the second configuration, as shown for example in FIG. 1D, the top corners of the rectangular bodies 102 and 104 impinge the palate tissue 114. Further, the front left and right corners of the bodies 102 and 104 in a transverse view may further impinge the palate tissue. This impingement is particularly problematic for patients with deep and narrow palates, and for large amounts of expansion between the bodies, for example greater than 10 mm. If discomfort is severe, the expansion screw may be removed temporarily, which may prolong treatment due to the interruption caused by the removal.
Therefore it is desirable for an expansion screw to be able to expand in deep and narrow palates and/or expand greater than 10 mm without impinging corners of the bodies into palate tissue.